One of the most common earth moving machines used in the general construction industry is the mechanized shovel. These machines are generally available in two varieties which are known as the "excavator" and the "backhoe." An excavator is generally the larger of the two machine types. A simplified drawing of a machine of this type is shown on FIG. 1.
An excavator, generally designated by the reference numeral 10, is usually a tracked machine with a pivot between its lower tracked carriage 18 and its cab assembly 20, which provides for side-to-side motion during operation. The digging apparatus generally consists of two extending members called arms, and a bucket 16. The first arm 12 is commonly called the "boom" and the second arm 14 is commonly called the "dipperstick."
A backhoe is generally smaller than an excavator but shares several similarities. A backhoe is generally a rubber tired machine which has its shovel portion on one end of the machine and another bucket on the other end. This second bucket apparatus is similar to a front-loader and is commonly used for moving material instead of digging. Like the excavator, the backhoe has a shovel implement which typically consists of a boom, dipperstick, and a bucket. This type of machine typically has a pivot between the cab portion and the boom arm to provide side-to-side motion while digging.
In operation, an operator of either a backhoe or excavator (hereinafter referred to generally as an "excavator") typically must dig to a particular elevation. If he digs too shallow then he must come back and rework the area; if he digs too deep then excessive fill material must be used. In order to determine the elevation in conventional systems, a second person is used to measure elevations for the machine operator. This person would either be using a laser system or an automatic level to determine the current elevation. If an automatic level is used then a third person is required to operate the level. This is further complicated if the digging depth is desired to be sloping, and not dug to a consistent (i.e., level) elevation. In this case, someone must keep track of the distance moved and periodically either add or subtract a certain elevation based on the distance and the desired slope.
Since some of the conventional digging systems in the prior art are so cumbersome and labor intensive, as described above, it would be very desirable to have available a digging system where the excavator operator can check his own digging elevation without the need for another person's help, and without stopping and getting out of the cab. Therefore, there is a need for an elevation indication system for excavators which is low cost, easy to install, is based on an absolute elevation reference, and if possible, provides elevation information while the operator is actually digging, rather than requiring him to stop to take a reading.
One major improvement in excavator systems is the use of a laser receiver mounted on the dipperstick of the excavator, in which the laser receiver intercepts the pulsed plane of laser light that is emitted by a rotating laser light source. Naturally, the more accurate the laser receiver, the greater the possible accuracy of the operation of the excavator. Therefore, a key element of many excavator systems is the ability of the laser receiver to operate with acceptable accuracy and in varying lighting conditions. To accomplish this function, a "long" laser receiver, generally designated by the reference numeral 30, is mounted on the dipperstick 14 of the machine 10 to accurately measure the position of the laser beam striking the receiver.
One way for a laser receiver to function at a wide variety of distances, and with different laser wavelengths, beam power, spot size, and spot shapes is to use a photocell configuration known commonly as a "split cell." FIG. 2 shows an example of a conventional photocell 60, for example, which is 12 inches (12"=30.5 cm) tall and 0.2 inches (0.2"=0.5 cm) wide in a split cell configuration (which is also known as a "dual triangle"). One of the triangles is designated "A" and represented by the reference numeral 62, while the other One of the triangles is designated "B" and represented by the reference numeral 64. It can be seen that the total surface area of cell "A" represented by the triangle 62 is equal to the total surface area of cell "B" represented by the triangle 64.
The split cell configuration is now well known in the art, and is commonly used to measure the position of light as described in U.S. Pat. No. 4,676,634 (by Peterson) and in other patents and product literature. This split cell configuration provides a linear indication of beam position that is independent of beam size, power, shape, etc. Using such a device, the beam position can be determined using the following equation:
Equation #1 ##EQU1## Using the Equation #1 to indicate position in inches, with twelve inches (12") at the top of the cell and zero inches (0") at the bottom, the following constants can be used:
K.sub.1 =12 PA1 K.sub.2 =0 PA1 K.sub.1 =12 PA1 K.sub.2 =10 PA1 K.sub.3 =8 PA1 K.sub.4 =6 PA1 K.sub.5 =4 PA1 K.sub.6 =2 PA1 K.sub.7 =0
This reduces Equation #1 to:
Equation #2 ##EQU2##
The configuration of FIG. 2 could theoretically perform the function described, however, it suffers from a variety of flaws and limitations. One very important problem is that it cannot be manufactured using existing manufacturing methods for silicon photocells. Presently, the largest silicon wafers used to manufacture single crystal silicon photocells are four inches (4"=10.2 cm) in diameter. Obviously it is impossible to manufacture a twelve inch (2") device from a 4" wafer.
In order to manufacture a photocell-based laser receiving device using existing process technology, the desired 12" long pattern can be divided into several lengths that are manufacturable. FIG. 3 shows a conventional 12" photocell configuration 70 where the 12" pattern has been broken into six portions of two inches (2"=5.1 cm) length each, thus creating a composite cell. The 2" length of each portion was chosen arbitrarily, however, it is roughly the limit for economical manufacturing of silicon photocells using existing processes. Each of the vertical portions on FIG. 3 comprises part of the photocell "A" and part of the photocell "B." To create a complete assembly, the vertical portion split areas are appropriately connected electrically, as follows: for cell "A" the split areas 71, 72, 73, 74, 75, and 76 are connected together, and for cell "B" the split areas 81, 82, 83, 84, 85, and 86 are connected together. It can be seen that the total surface area of cell "A" represented by the separate split areas 71-76 is equal to the total surface area of cell "13" represented by the separate split areas 81-86.
In the configuration of FIG. 3, three different cell patterns must be tooled to manufacture the composite cell, which will add to the tooling cost of the product. Furthermore, since the overall pattern for the device is 12" long, the sensitivity of system resolution and accuracy to noise and component tolerances is high. Existing products using this configuration for rotating laser beam detection have achieved 0.01" (0.025 cm) accuracy and resolution with a 2" long cell configuration. This 12" long configuration will be degraded by at least at factor of six. Since the split cell shapes are substantially symmetrical, it can be seen that the top cell portion 71, 81 is virtually identical in construction to the bottom cell portion 76, 86. The same is true for the central cell portions 73, 83 and 74, 84, and the intermediate cell portions 72, 82 and 75, 85.
A further improvement can be made to the photocell configuration, as shown in FIG. 4. By breaking the 12" length of the photocell assembly 100 down into a series of 2" long split cells and connecting them together, one composite photocell array can be formed. Each of the vertical portions on FIG. 4 comprises two triangular portions that are individually treated in the Equation #3 below, with the modifier that certain adjacent triangular areas are electrically connected together. For example, triangle "A" at 101 is not electrically connected to any other similar triangle, whereas there are two triangles "B" at 102 and 103 that are electrically connected together. Other paired triangles on FIG. 4 are 104-105 (cell "C"), 106-107 (cell "D"), 108-109 (cell "E"), and 110-111 (cell "F"). The final triangle "G" at 112 is an electrically independent cell.
The surface area of cells A (at 101) and G (at 112) in FIG. 4 are substantially equal to each other. Likewise, the combined surface area of paired cells B (at 102 and 103) are substantially equal to the combined surface area of the other paired cells on FIG. 4, i.e., cells C (at 104, 105), cells D (at 106, 107), cells E (at 108, 109), and cells F (at 110, 111). Furthermore, the combined areas of cells A (at 101) and G (at 112) are substantially equal to each of the paired cells B-F.
The laser beam position of the photocell configuration illustrated in FIG. 4 is determined by the following Equation #3:
Equation #3 ##EQU3## Once again to indicate position in inches, with 12 at the top and 0 at the bottom, the following constants could be used:
The use of the above constants reduced Equation #3 to:
Equation #4 ##EQU4##
This configuration of FIG. 4 can be manufactured using existing processes and will function with good accuracy and resolution, but still suffers from an important problem. This problem is called "rotational offset," which occurs when the photocell array is not directly facing the laser transmitter. In this situation, the housing that contains the photocell array may shade a portion of the photocell. If partial shading occurs, one side of the photocell may not receive any laser energy while the other side may receive full energy. The result in the laser receiver system is that the calculated position derived from the photocell outputs will be in error. And this could be as much as 2" of error in the laser receiver system described above.
In order to examine this phenomenon, a single photocell 120 having a 2" split cell pattern is depicted in FIG. 5. In this illustration, a laser beam is depicted at the circle 130 as striking near the top of the photocell 120 and the indicated line shading portion of the drawing (to the left of the vertical line 132 on FIG. 5) depicts the physical area 134 of the photocell that may be shaded from the laser light source due to the relative positioning of the receiver housing. Without the shading effect, the majority of the laser beam spot 130 would impact the surface of the triangular split cell portion at 122, as compared to the amount of the laser beam spot 130 that would impact the surface of the triangular split cell portion at 124. However, because of the shading effect, this will not be true, and a large error due to rotational offset will occur.
It is clear from FIG. 5 that, instead of the majority of the laser energy striking the top portion of the photocell in the correct proportions as described in the previous paragraph, the laser beam spot will impact an area of the split cell portion 124 at the reference numeral 125, which is fairly close to being the proper amount of surface area. Unfortunately, the laser beam spot will impact an area of the split cell portion 122 at the reference numeral 123 which is greatly reduced in surface area as compared to the proper amount to register a correct spot position reading. Because of the shading, there are nearly equal parts of the laser beam striking the upper and lower portions 122 and 124. The result of this shading effect is that there is a direct position error generated by the positioning of the housing that cannot be either detected or corrected for when using this photocell design.
This shading problem is well known in the laser receiver industry and has been described and partially corrected in U.S. Pat. Nos. 4,907,874 and 4,976,538 (by Ake). In U.S. Pat. No. 4,907,874, the configuration described suffers from none of the rotational offset problem, however, it does suffer from linearity problems. This linearity problem makes this particular design unsuitable for handling the variety of laser beam sizes and shapes required in the construction laser industry.
In U.S. Pat. No. 4,976,583, the linearity problem is corrected with a slight penalty of re-introducing a small amount of rotational offset. The main penalty associated with the configuration of U.S. Pat. No. 4,976,583 is that, since its photocell pattern is very complex, it is difficult and expensive to manufacture. Also, since the 2" length of the cell is roughly at the limit of what can be manufactured, only a few manufacturers can produce the part, and the manufacturing yield and thus cost is negatively impacted. Further, since it is so complex, it is not easily stacked on end, as opposed to the photocell assembly described in FIG. 4. It is desirable, therefore, to find a simpler photocell configuration that can be stacked on end to create a composite array of any arbitrary length.
FIG. 6 illustrates in improved conventional photocell 140 that is constructed in a two-split cell configuration (of three cell triangles 142, 144, and 146 ) where the split cells are arranged side-by-side. This configuration reduces the amount of rotational offset that will be produced by cell shading up to the point at which only a small portion of the cell is left unshaded. However, it would be a further improvement to provide a photocell arrangement that reduced the error due to rotational offset to an even greater extent, so that rotational offset error is largely corrected when only an extremely small portion of the photocell is unshaded.